It follows from singular perturbation theory [1]}, [2]}, [3]} that the solutions of system () with appropriate initial
conditions converge for \(\delta \rightarrow 0\)  to the solutions of the differential-algebraic system
\(0 &=& - k_1 x_1 (k_4 - x_4 + x_1) + k_2 (x_4-x_1), \\x^{\prime }_4 &=& k_3 (x_4 - x_1),\) 
